The present invention relates to bookbinding machines in general, and more particularly to improvements in bookbinding machines of the type referred to as perfect binders.
It is known to combine the tongs of a perfect binder into an endless chain and to provide each tongs with two clamping jaws one of which is movable relative to the other so as to clamp a stack of sheets during transport of the stack along one or more treating stations. The tongs are pushed along an endless path, and the means for biasing the one jaw toward the other jaw normally comprises springs. Reference may be had to DE-AS 1 262 221.
A drawback of conventional perfect binders is that, if the tongs are not extremely rigid and bulky, the movable jaws are likely to jam during clamping of relatively small stacks of overlapping sheets. Any jamming of movable jaws in their tracks entails a non-uniform distribution of stresses along that edge of the corresponding stack which is clamped between the book-engaging portions of the respective jaws. This affects the quality of the ultimate products.
Attempts to overcome such drawbacks of conventional perfect binders include the utilization of extremely heavy and bulky tongs which, in turn, necessitates the provision of heavy and sturdy tracks for the followers of frames which carry the jaws of the respective tongs. Moreover, such heavy and bulky tongs cannot be readily transported in stepwise fashion at elevated speeds. Relatively heavy and bulky tongs cannot be accelerated and/or decelerated within short intervals of time.
A further drawback of conventional bookbinding machines of the type known as perfect binders is that each of the tongs must be provided with a substantial number of relatively heavy and strong springs which urge the book-engaging portions of movable jaws toward the book-engaging portions of the complementary jaws. This can be accomplished only if the tracks for the tongs are machined with a high degree of precision and the followers of the frames forming part of the tongs are guided in such tracks with a minimum of play. All this contributes to the initial and maintenance cost of conventional perfect binders.